1. Field of the Invention
The present invention relates to a method and apparatus for drying a pulverulent or particulate product in a fluid bed dryer with back mixed solids flow.
When a wet product is to be dried in a fluid bed dryer of the said type the amount of heated fluidizing gas supplied to the fluid bed is advantageously kept at a minimum necessary to fluidize the product, and additional heat is then transferred to the fluidized product by means of heating surfaces submerged in the fluidized product layer.
For economic reasons, it is desired to maintain the temperature of the fluidizing gas discharged from the fluid bed and, consequently, also the temperature of the product being dried, substantially at the dew point. To obtain a uniform product of controlled properties it is important that the product discharged from the fluid bed has a degree of moisture which is substantially constant even when the rate of feeding the product to the fluid bed as well as the conditions of the product feed, such as degree of moisture and temperature, vary within certain limits. This may be obtained by controlling the heat transferred to the wet product from the heating surfaces submerged in the fluidized product.
2. Description of the Prior Art
In a known drying method of the above type, the heat transfer from the heating surfaces to the wet product is controlled by the operator so as to maintain the temperature of the drying gas discharged from the fluid bed at a predetermined value. This predetermined value which is calculated to be slightly above the dew point of the discharged fluidizing gas (for example by about 5.degree. C.) is based on an estimated degree of moisture of the product and on an expected rate of feeding the product to the bed. The expected level of the discharge gas temperature should secure that all free surface moisture has been evaporated from the product when the product leaves the fluid bed. However, in practice the moisture and/or the feeding rate of the product may substantially exceed the expected values and to such an extent that the product cannot be maintained in a fluidized condition, whereby the drying process is discontinued, or the moisture and the feeding rate may be less than expected, whereby the heat economy of the drying process is reduced. In any case, it is not possible to obtain a uniformly, dried product.